This invention relates to an actuator device for use as, for example, a driving section of a flow rate control valve of a flow rate control apparatus.
Generally, a conventional actuator device of the type comprises an actuator element which has a shortest length when no electric voltage is applied thereto and which is increased in length when an electric voltage applied thereto becomes higher, a displacement extracting part, and a converter mechanism for converting the change in length of the actuator element into the displacement of the displacement extracting part.
As will later be described, the converter mechanism of the conventional actuator device carries out conversion so that the displacement of the displacement extracting part is increased as the actuator element is increased in length. Thus, the conventional actuator device has a structure such that, as the electric voltage applied to the actuator element becomes higher, the length of the actuator element is increased so that the displacement of the displacement extracting part becomes greater.
It is assumed here that the actuator device is used as a driving section of a flow rate control valve of a flow rate control apparatus and that the actuator element is not supplied with the electric voltage because a power supply to the apparatus is interrupted by any accident (i.e., the actuator element is applied with a zero voltage). In this event, the length of the actuator element is shortest as mentioned above so that the displacement of the displacement extracting part is smallest. In response to the displacement of the displacement extracting part, the flow rate control valve puts a flow path into an open state.
As a result, when the power supply to the apparatus is interrupted by any accident, a large volume of a fluid such as a gas continuously flows through the flow path.
In order to eliminate the above-mentioned disadvantage, it is proposed to invert the displacement of the displacement extracting part of the actuator device by the use of an external mechanism separate from the converter mechanism. In this event, however, the actuator device inevitably becomes large in size.